Adhesive bonding of structural components provides several advantages over mechanical fastening. For example, adhesive bonding results in a more uniform stress distribution across a bonded joint relative to the stress distribution across a mechanically-fastened joint. In addition, adhesive forms a barrier between structural components which may avoid undesirable effects that may be associated with mechanical joints wherein dissimilar materials are in direct con tact with one another. Adhesive bonding of structural components may also reduce assembly costs and provide increased fatigue life relative to the assembly costs and fatigue life associated with mechanically-fastened joints.
A fillet bond is a type of adhesive bond between a curved surface of one component and a generally flat surface of another component. In a fillet bond, adhesive material forms fillets between the joined components. The thickness of a fillet bond may vary across a bondline between the two components. Advantageously, fillet bonds may provide improved stress distribution in a joint by spreading load in the joint over a relatively large surface area.
For certain assemblies having bonded joints, it may be necessary to qualify the strength- carrying capability of the bonded joints prior to placing the assembly into service. For example, prior to certifying an aircraft, it may be necessary to qualify bonded joints in primary load-carrying structure of the aircraft. Qualifying the bonded joints may include verifying that the margins of safety of the bonded joints are within design limits. The margins of safety may be determined by applying loads to test specimens of the bonded joint wherein the loads simulate actual loads to which the bonded joint may be subjected during the service life of the joint.
Certain structures may be subjected to in-service loads that place a fillet bond in tension pulloff (e.g., out-of-plane) loads. One such structure that may be subjected to tension pulloff loads is a skin panel having a plurality of stiffeners positioned in spaced relation to one another on one side of the skin panel. Each one of the stiffeners may be bonded to the skin panel with a fillet bond extending along a length of the stiffener. During service, the tension pulloff load may be distributed generally uniformly across the skin panel and may cause the fillet bonds to be loaded in tension.
Conventional testing apparatuses for testing the load-carrying capability of bonded joints are generally directed toward flat-bond geometry between two generally planar components. Adhesive material in such flat-bond geometry may have a generally uniform thickness across the bonded joint. Conventional testing apparatuses may include a generally flat backing plate that may be mounted to an outer surface of a test specimen having a flat-bond geometry. The testing apparatus may be mounted within a testing machine. The test specimen may be instrumented with strain gauges so that strain measurements may be recorded during application of a test load. The strain levels may be converted to stress. The test specimen may be tested to failure and the stress levels at failure may be correlated to strength values of the bonded joint.
Unfortunately, conventional testing apparatuses using backing plates may not provide an accurate duplication of the forces induced in a fillet bond. For example, for the above-mentioned structural assembly having stiffeners bonded to a skin panel, tension pulloff loads on the skin panel may induce bending loads in the test specimen at the fillet bonds. Although conventional testing apparatuses may be adequate for applying a tension test load to a fillet bond in a test specimen, the stiffness of the backing plate prevents the generation of bending loads in the test specimen.
A further drawback associated with conventional testing apparatuses is that eccentric or asymmetric loading may be produced in a test specimen. Such asymmetric loading may be caused by manufacturing tolerances of the test specimen, by misalignment of the test specimen with the testing machine, or due to other factors. The occurrence of asymmetric loading may minimize the repeatability of testing conditions across a plurality of test specimens and may compromise the accuracy of test results.
As can be seen, there exists a need in the art for a testing apparatus and method that may substantially duplicate the loading conditions in a fillet bond during application of a tension to the fillet bond. In this regard, there exists a need in the art for a testing apparatus and method capable of accurately inducing symmetric bending in a fillet bond during application of a tension test load. Ideally, the testing apparatus may avoid the need for measuring strain in a test specimen and then correlating the strain to stress for determining the strength capability of the fillet bond.